Gaseous fuel powered engines are common in many applications. For example, the engine of a locomotive can be powered by natural gas (or another gaseous fuel) alone or by a mixture of natural gas and diesel fuel. Natural gas may be more abundant and, therefore, less expensive than diesel fuel. In addition, natural gas may burn cleaner in some application, and produce less greenhouse gas.
Natural gas, when used in a mobile application, may be stored in a liquid state onboard the associated machine. This may require the natural gas to be stored at cold temperatures, typically about −100 to −162° C. The liquefied natural gas is then drawn from the tank by gravity and/or by a boost pump, and directed to a high-pressure pump. The high-pressure pump further increases a pressure of the fuel and directs the fuel to the machine's engine. In some applications, the liquid fuel may be gasified prior to injection into the engine and/or mixed with diesel fuel (or another fuel) before combustion.
One problem associated with pumps operating at cryogenic temperatures involves flash boiling of the natural gas due to low pressures observed during retracting strokes of the pump's pistons. In order to avoid such low pressures, and thereby avoid flash boiling of the natural gas, typical cryogenic pump systems either incorporate large-diameter slow-moving pistons located at the bottom of a fuel tank to minimize pressure, or the systems include an additional boost pump that elevates a pressure of the fluid being directed to the pistons of a separate main pump. Using large diameter pistons results in large, heavy, and expensive pumps that create high-pressure spikes in downstream components (e.g., in accumulators that collect fluid from the pumps). The pressure spikes can be complex and expensive to accommodate (e.g., requiring additional components, such as regulators). Incorporating an additional boost pump can increase a cost of the pumping system and also reduce a reliability of the system.
An exemplary pump is disclosed in U.S. Pat. No. 5,464,330 (the '330 patent) that issued to Prince et al. on Nov. 7, 1995. In particular, the pump of the '330 patent includes a tank and three cylinder blocks disposed in the tank. Each cylinder block has a first stage cylinder and a second stage cylinder, with an associated piston disposed in each of the first and second stage cylinders. A crankshaft extends into the tank and includes an eccentric, lobe against which the first stage piston is biased. The second stage piston is free floating in the second stage cylinder.
During a suction stroke of the first stage piston of the '330 patent, hydraulic oil is sucked into the first stage cylinder past a first check valve. During a compression stroke, the first stage piston is driven to force the hydraulic oil out of the first stage cylinder past a second check valve into a passageway that is common to each of the cylinder blocks in the tank. During a suction stroke of the second stage piston, hydraulic oil from the passageway is drawn into the second stage cylinder past a third check valve. During a compression stroke of the second stage piston, the hydraulic, oil is driven to force the hydraulic oil out of the second stage cylinder past a fourth check valve into an outlet that is common to all of the cylinder blocks. A pressure of the fluid in the passageway is sufficient to move the second stage piston to its retracted position as the first stage piston is spring-biased to its retracted position.
While the pump of the '330 patent may be useful in some hydraulic oil applications, it may have limited applicability in cryogenic applications, in particular, the pressures experienced in cryogenic applications could be high enough to cause distortion of the cylinder blocks of the '330 patent. In addition, the passages and check valves of the '330 patent may not be sized properly for cryogenic applications, potentially causing flash boiling during the retracting strokes of the first and second stage pistons.
The disclosed pump is directed to overcoming one or more of the problems set forth above.